Vessels that hold corrosive fluids such as strong acids or strong bases must be made of materials that resist corrosion; suitable materials include polyethylenes and polyfluorocarbons. High-density polyethylene is an especially useful material because it is relatively inexpensive compared to specialty plastics such as fluorocarbons and is also resistant to both acids and bases. Further, it is available in preparations that are free of contaminants, such as trace metals, that can react with corrosive chemicals and thereby compromise their purity and also weaken the structural integrity of the vessel. High-density polyethylene is also amenable to conventional plastics processing techniques so that vessels such as drums, etc., may be made.
Fluids are moved in and out of these vessels via ports, which are openings in the vessel. Vessels that are used to store corrosive chemicals must have ports that can be sealed so that the vessels can be sealed as they are moved from one location to another or are used to store the fluids. Thus a port on the vessel typically receives an insert that achieves a seal between the port and the insert. A port should be sealed not only for the proper storage of corrosive fluids but also not leak when it is fitted with a device that allows transfer of fluids in or out of the vessel, for example, a transfer tube that allows the vessel's contents to be moved out of the vessel.
Vessels used for highly corrosive fluids conventionally require encapsulated dual material O-rings to achieve a seal between the vessel's port and an insert. Encapsulated O-rings are conventionally equipped with a core made of a resilient, flexible material and an outer layer made from a different material that provides corrosion resistance. Thus, the resilient core provides resiliency and flexibility while the outer layer resists corrosion.
Flexibility allows for an O-ring to be deformed so that it may be readily handled and provides a larger sealing area. Thus a flexible, donut-shaped O-ring with a circular cross-section can be compressed and deformed so that its round edges become conformed to the shape of the sealing surface(s) that it contacts. Silicone rubber is a conventionally used core material that has suitable flexibility. Flexibility is related to durometer hardness.
A resilient material tends to resist stresses that cause it to deform from its original shape. Thus a resilient O-ring that has been compressed between two sealing surfaces will tend “push back” against those surfaces and maintain a seal. Further, a resilient material will compensate for expansion or contraction of the vessel, insert, and O-ring materials in response to factors such as temperature changes. Thus when the sealing materials contract in response to cold temperatures, a resilient O-ring will expand to maintain the seal. In contrast, a poorly resilient material will tend to permanently deform in response to compressive forces and forms a weaker seal. A poorly resilient material that gradually deforms after it has been compressed into place experiences plastic flow, also know as “creep”, and is likely to provide a poor seal as time passes. Silicone rubber is a conventionally used material that has suitable resiliency.
Silicone rubber, however, has poor chemical resistance and is unsuitable for use as a sealing material that contacts strong acids or bases. Further, it is typically manufactured with significant amounts of impurities that may compromise the purity of fluids in the vessels.
For all of these reasons, O-rings for use with corrosive fluids are conventionally made with a resilient material covered with an outer layer of corrosion-resistant material and are referred to as encapsulated dual material O-rings. The outer layer is conventionally a fluorocarbon polymer such as polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP), or perfluoroalkoxy (PFA). Fluoropolymers have outstanding corrosion resistance and are generally suitable for use with strong acids and bases. They are also impermeable so that the corrosive fluids do not flow through the O-ring and the impurities of the core layer are contained. Fluoropolymers are the conventionally used engineering plastics for making O-rings that resist corrosion and the release of impurities upon exposure to solvents.
Encapsulated dual material O-rings for drums having at least a 30 gallon capacity are made by an expensive and time consuming process and currently cost approximately $4.00 each. They are typically used as a single-use device so that they are replaced every time an insert is changed in the vessel's port. Their cost is substantial in light of the retail cost of a typical vessel for corrosive liquids, which is about $25.00 for a 55-gallon drum. The corrosive fluids industry would greatly benefit from the replacement of encapsulated dual material O-rings with an inexpensive alternative.
Despite the need for an alternative, the applicants are unaware of any conventionally available alternatives to encapsulated dual material O-rings for use with corrosive or critical fluids.